The United Nations last night appointed one of Africa's elder statesmen as a peace envoy to the Democratic Republic of Congo to mediate a deal between regional antagonists, as fears grew for the fate of tens of thousands of civilians driven into the bush by the latest upsurge in fighting.

According to diplomats at the UN headquarters in New York, the former Nigerian president, Olusegun Obasanjo, was assigned by the UN secretary general Ban Ki-Moon to work with the African Union to hammer out a working peace deal between the Congolese government, the Tutsi rebels in eastern Congo, and the Rwandan government which is widely believed to back them. Diplomats said last night that all sides had accepted Obasanjo as a mediator.

Among his first tasks will be to help the AU and its chairman, the Tanzanian president, Jakaya Kikwete, maintain momentum behind peace talks. Kikwete had expected a Rwandan government delegation in Dar-es-Salaam yesterday, but there was no report of its arrival. Face-to-face negotiations in Nairobi are planned within a week. Diplomats involved in talks over the weekend expressed concern that if movement towards a settlement were allowed to lag, a ceasefire negotiated last Wednesday could collapse.

Ban also reappointed a Senegalese general, Babacar Gaye, as head of the UN peacekeeping force in Congo. Gaye had commanded the force, Monuc, until last month, but his Spanish successor, Lieut Gen Vincente Diaz de Villegas, resigned last week for unspecified personal reasons.Obasanjo and Gaye's appointments came on a day when the scale of the looming humanitarian disaster became clearer. An aid convoy crossed the front line escorted by UN peacekeepers and carrying medicine and water for civilians forced from their homes by a rebel offensive led by Gen Laurent Nkunda, and looting by retreating Congolese government troops. When the convoy arrived at a relief camp for the displaced population around Rutshuru, a town in the province of North Kivu recently seized by the rebels, it found the camp had been destroyed.

Alice Gilbert, based in Rutshuru for the British medical charity, Merlin, said her colleagues on the convoy witnessed scenes of desolation. "It looked like a very strong wind had blown it over," Gilbert said. "It's all completely destroyed, levelled to the ground. There was a health post there; the only evidence of it now is a concrete stump."

The camp's population of 22,000 appeared to have melted away, together with several tens of thousands displaced by earlier fighting. Some may have taken refuge in homes of local residents. Others clearly had fled into the bush. "We're extremely worried about where these people are," she said. "We're worried there are a lot of people living out in forest with no food, water or medical supplies."

Britain announced yesterday that it was sending a flight to the region, with 18,000 blankets and plastic sheeting and water buckets. The Department for International Development estimates that there are 900,000 people displaced in North Kivu alone, of which 55,000 were forced out by last week's violence.

The UN force, Monuc, has been bitterly criticised by survivors of the latest violence for failing to uphold its mandate to protect civilians. Of the 17,000-strong force only 850 are in Goma. The head of UN peacekeeping, Alain Le Roy, assessed the force over the weekend, and yesterday recommended a reconfiguration to the UN. However the force's flexibility is hampered by "caveats" imposed by national troop contributors limiting the role of their soldiers.

Britain's foreign secretary, David Miliband, and his French counterpart, Bernard Kouchner, reported yesterday to their fellow European ministers in Marseille on their two-day trip to the region, during which they helped start regional peace talks and warned Congolese and Rwandan leaders that they would be held to account for any further fighting.

Although Miliband said use of European forces could not be ruled out, it was clear there was little interest among member states in dispatching soldiers.

Kouchner said the initial emphasis would have to be on a reorganised Monuc force, with enhanced rules of engagement: "They have helicopters. They have 83 planes ... the problem is perhaps to have a wholesale change of the organisation of Monuc ... they have a robust mandate."